Coated polymeric thermoplastic dielectric film



COATED POLYIVIERIC THERMOPLASTIC DIELECTRIC FILM N Drawing. ApplicationMay 22, 1956 Serial No. 586,359

17 Claims. (Cl. 117138.8)

This invention relates to a process of improving the dielectric life ofpolymeric thermoplastic materials useful as electrical insulation and tothe product of such process and, more particularly, to a coatedpolyethylene terephthalate film having an improved dielectric life.

Polyethylene terephthalate film, particularly oriented film (film whichhas been stretched and/ or rolled in two directions and heat-set atelevated temperatures within the range from 150-250 C.) has been foundto possess a unique combination of electrical, physical and chemicalproperties which make it outstanding for use as a dielectric. That isparticularly true of polyethylene terephthalate film which has beenstretched and/ or rolled to substantially the same degree in bothdirections, e.g., 3X, where X equals the original dimension of the film,to obtain a substantially balanced film, which means that the physicalproperties of the film, as measured in both directions, aresubstantially the same. The oriented, heat-set, polyethyleneterephthalate film is outstanding as a dielectric because it retains aconsiderable percentage of its electrical properties, strength, anddurability at elevated operating temperatures. Consequently, the film isparticularly useful as a dielectric in capacitors, motors, generators,transformers, etc., at not only moderate operating temperatures, butalso at ambient temperatures approaching 150-l75 C.

In extending the usefulness of polyethylene terephthalate and likethermoplastic polymer film dielectric to a wider variety of electricalend uses, it became apparent that the dielectric life of the filmrequired improvement, particularly for use in equipment subjected tohigh voltage stresses. As employed herein, the term dielectric life ofthe polymeric thermoplastic film applies to the actual time that aparticular film may be subjected to a particular voltage stress underconditions of corona discharge before actual physical breakdown, i.e.,rupture, of the film dielectric.

It is the action upon the polymeric film dielectric of corona discharge(in air or other gaseous medium) which causes actual physical breakdownof the film under conditions of a particular voltage stress (a potentialdifference large enough to produce a visible discharge but not largeenough to produce instantaneous breakdown). Obviously, the lower thevoltage stress, the longer the dielectric life of the film. Coronadischarge? is defined as the discharge of electricity which appears uponthe surface of a conductor when the potential gradient exceeds a certainvalue, For example, when a continuous potential applied to a pair ofnarrow wires is slowly incre'asedfthe voltage will'be reached at which ahissing noise is heard, and a pale violet light is visible (in the dark)around the'wir'e'sl" This'voltage is defined as the critical visualcorona point. Corona is due to ionization of the air or surroundinggases (it does not occur in a vacuum). That is, the air in the ionizedregion is conducting, and this results in increasing the effectivediameter of the conductor. For example, even though United States Patenta film dielectnc is wound tightly around a wire, air is not entirelyexcluded from the space between the conductor and the film dielectric;and it is the air or other gaseous medium which is ionized. The coronaenvelopes a conductor as a concentric cylinder, and the outside diameterbecomes such that the gradient at that point decreases to the rupturingpoint of the air. Beyond this the film eventually ruptures; hence, thiscauses short cir-' cuits owing to the physical failure.

An object of the present invention is to provide a polymericthermoplastic dielectric film having increased dielectric life. Afurther object is to provide a polyethylene terephthalate filmdielectric having increased dielectric life. A further object is toprovide a process of improving the dielectric life of polymericthermoplastic dielectric compositions, particularly polyethyleneterephthalate film. A still further object is to provide a novel coatingcomposition useful for improving the dielectric life of polymericthermoplastic dielectric film. Other objects will be apparent from thefollowing description of the invention.

These objects are realized in accordance with the present inventionwhich, briefly stated, comprises providing a dielectric base film ofthermoplastic polymeric material, e.g., polyethylene terephthalate, witha relatively thin, adherent, continuous coating consisting essentiallyof a homogeneous mixture of from 95% to 40% by Weight of (A) a curedresinous compound selected from the group consisting of (l) phenolformaldehyde resins, (2) alkyd resins resulting from the inter-reactionof a polybasic carboxylic acid and a polyhydric alcohol, (3)

long chain fatty acid-modified alkyd resins, and (4) the condensationproduct of epichlorohydrin and bis p,p'- hydroxyphenyldimethylmethane,and from 5% to 40% by Weight of (B) finely divided particles from thegroup consisting of amorphous silica and estersils, said particleshaving an ultimate average particle size between 5 millimicrons and 5microns.

The present invention will be further described specifically withrespect to improving the dielectric life of polyethylene terephthalatefilm but it should be understood that the invention is applicable asWell to enhancing the dielectric life of other types of unplasticizedpolymeric thermoplastic films useful as dielectrics, such as fihns ofpolyethylene, polytetrafiuoroethylene, polystyrene, polyamides, etc.

Any phenol-formaldehyde resin is operable in this invention; It has beenfound that both heat-reactive and non-heat-reactive resins (i.e.,thermosetting resins and non-thermo-setting resins) and bothoil-reactive and nonoil-re'active resins are'operative. The term phenolincludes beside phenol itself, phenol derivatives such as cresol,paraphenylphenol, xylenol, ethyl phenols, salicyclic acid, andresorcinol and chlorinated phenols. The oilreactive phenols may bemodified by the addition of up to by weight of a drying or se '-dryingoil. Preferably, the oil is added to a solution of the resin in avolatile organic solvent, e.g., toluene, xylene, mineral spirits, etc.

The alkyd resins utilized in the coating compositions characterizing thepresent invention may be prepared in known manner by inter-reacting apolycarboxylic acid or the anhydride thereof, having from 2 to 3carboxyl of from 15% toj8 0% of a higher fatty acid, i.e., a 1

fatty acid having at least 8 carbon atoms. The fatty.

acids are preferably selected from the group comprising unsaturated dryoil type fatty acids, e.g., coconut oil acid,

and saturated non-drying monocarboxylic fatty acids, e.g., Z-ethylhexoic acid.

The epoxide resins employed in this invention are formed by thecondensation of epichlorohydrin andbisp,p'-hydroxy-phenyldimethylmethane in the presence of sodiumhydroxide. During the condensation HCl or NaOl is split out to formpolyethers which have epoxy groups on the end of the chain. The degreeof polymerization is expressed in terms of epoxy equivalents. The termepoxy'equivalent is defined as the weight of the resin in grams whichcontains 1 gram equivalent Weight of the epoxy groups. The resins of theabove type are available commercially. One brand'of'such resins is soldby the Shell Chemical Corporation under the name Epon.1 7

Types of finely divided particles which are to be blended with the blendof alkyd or phenolic resins are selected from the following compounds:(1) amorphous silica, and (2) estersils as defined in U.S. Patent2,657,149. The essential requirement is that whenever any of the abovecompounds are blended with the alkyd, phenolic, or epoxy resinscharacteristic of the coating compositions of the present invention, thecompound must be in finely divided form such that the ultimateparticles, either in the form of super-colloidal aggregates or.individual particles, have an average greatest dimension ranging between5 millimicrons and 5 microns, and preferably below 50 millimicrons. Sometypes of the above compounds may not be commercially available in finelydivided form wherein the average particle size is 5 microns or less. Insuch cases, it is necessary to process the particles by ball-milling orother techniques to produce the size of the average particle.

The commercially available types of amorphous silicas may besatisfactorily employed in the present invention.

These silicas may be arbitrarily divided into four 'classifications,namely: aerogel, precipitated, vapor phase, and diatomaceous types.Santocel CS, manufactured by the Monsanto Chemical Company is an exampleof the aerogel type. This is slightly acidic and contains a small amountof alcohol, water and sodium sulfate as the chief impurities. Theparticlediameter is approximately 30 millimicrons, with a surface areaof approximately 100-15O mi /gm. (squme meters per gram). Theprecipitated silicas may be represented by Hi-Sil X-303 or Hi-Sil XW,manufactured by the Columbia-Southern Chemical Corporation' Thismaterial has an average 7 particle diameter of 20-25 millitnicrons and asurface area within the range of 140-160 m. /g'm. A typicalrepresentative of the vapor phase type of silica is Aero sil,manufactured by Godfrey L. Cabot, Inc. This material has an averageparticle diameter approximately 14-20 millimicrons and a surface area of175-200 mF/gm. The diatomaceous types of silicas may be further dividedinto two groups which are important in compounding the present coatingcompositions. These are the heat-calcined, and flux-calcined materials.Both are relatively free from organic contaminants. Particle sizesare'mnch larger than those of the silicas mentioned above, ranging fiom1-6 microns. The surfaces areas are larger than would be calculated fromparticle diameter owing to high internal area in this form of silica. Inthe heat-calcined group are such products as Celite 270, manufactured byJohns-Manville Corporation and Dicalite PS, manufactured by Great LakesCarbon Corporation. In the flux-calcined group are Dicalite White andCelite Superfloss.

The estersils useful for purposes of this invention are 4 described inU.S. Patent 2,657,149 to Ralph K. Iler. In brief, the estersils areesterified super-colloidal substrates, that is, a substrate in the formof particles coated with OR groups, the substrates having a surface ofsilica and having a specific surface area of at least 1 m. /gm., thecoating of -OR groups being chemically bound to said silica, and R beinga hydrocarbon radical of at least 2 carbon atoms wherein the carbon atomattached to oxygen is also attached to hydrogen. In general, theestersils have a specific area of at least 1 m. /gm., usually within therange of l-900 m.'-/gm.

The coating compositions .may conveniently be prepared by firstdissolving the resin component ina suitable volatile organic solventsuch as toluene, xylene or mineral spirits, and ball-milling theresulting solution admixed with the finely divided particles of silicaor estersil to form a homogeneous blend. The composition should containfrom 10 to 60%, and preferably from 20 to 40% by weight of total solids.

The coating composition may be applied to one or both surfaces,preferably both surfaces, of the base film by any desired expedient; andthe coated film may thereafter be dried at room temperature ormoderately elevated temperatures to remove solvent. Curing of theresin/particle blend to harden the resin should be carried out atelevated temperatures, e.g., l00-l75 C., for durations from 10 minutesto 60 minutes. In some instances, air-drying at room temperature, issufiicient to effect hardening of the coating.

It may be necessary, depending upon the end use for the coateddielectric films of the present invention, to apply a subcoating to thebase dielectric film, e.g., oriented, heat-set, polyethyleneterephthalate film, to improve adhesion between the base film and theresin/ particle coating. When applying the subject coating compositionsto polyethylene terephthalate film, for example, it is preferred toapply a subcoating which is chemically similar to the base film. Anysuitable subcoatings may be employed provided that they improve theadhesion betweenthe base film and the present coating and do notadversely affect the original combination of physical, electrical andchemical properties of the base film. Preferred subcoatings forpclyethyleneterephthalate film include copolyesters derived by reactingglycol, terephthalic acid, or low alkyl ester thereof with a second acid'or alkyl ester thereof from the group consisting of sebacic acid,isophthalic acid and hexahydroterephthalic acid. Normally, in preparingsuitable subcoatings, it is preferred that the subcoating compositionEXAMPLE 1 The following formulation:

Parts Super-Beckacite 1003 (Thermosetting varnish oil reactive-soluble,phenol formaldehyde resin- Reichold Chemical Company) -L. 75 Estersil(as described in U.S. Patent 2,657,149) 20 Toluene V 173 was mixed(35.5% solids) and placed in a ball-mill apparatus. The contents wereagitated for 116 hours. Polyethylene terephthalate (0.5 mil inthickness) was coated with this composition and air-dried at roomtemperature.

The film was then cured in an oven at C. for 10 minutes.

was mixed (35.3% solids) and agitated in a ball-mill apparatus for 72hours. Polyethylene terephthalate film (0.5 mil in thickness) was coatedwith this mixture and air-dried at room temperature. cured in an oven at150 C. for 10 minutes.

EXAMPLE 3 The following formulation:

Parts Glyptal 2502 (60% solvent solution-glyceryl 'phthalate having highhydroxy content-Gen eral Electric Company) 110.6 Toluene 109 Cab-O-Sil(finely divided silica-Godfrey L.

Cabot, Inc.) 27

was mixed (37.7% solids) and agitated in a ball-mill apparatus for 168hours. This mixture was applied to 0.5 mil thick polyethyleneterephthalate film and the film was air-dried at room temperature. Thecoated film was then cured in an oven at 150 C. for 10 minutes.

EXAMPLE 4 The following formulation:

Parts Epon (epoxide resinAhell Chemical Company) 40 Estersil (asdescribed in US. Patent 2,657,149) 10 Toluene 100 Methyl ethyl ketone100 was mixed solids) and agitated in a ball-mill apparatus for 240hours. Polyethylene terephthalate film (0.5 mil in thickness) was coatedwith this mixture and air-dried at room temperature. The coated film wascured in an oven at 150 C. for 10 minutes.

The coated film was" thermoplastic film applies to the actual below.

was mixed (37.7%) and agitated in a ball-mill apparatus for 168 hours.This mixture was applied to 1.5 mil thick polyethylene film and the filmwas lair-dried at room temperature. The coated film was then cured in anoven" at 75 C. 'for 20 minutes.

Table I, below, records the corona life of the coatings described inExamples l5, inclusive. The fifth failure in 10 samples was taken asrepresentative of possible" corona life. of the coated Also listed areoverall thickness (gauge) under Examples sides of the film. I, weresubjected l-5, inclusive, were coated on both The coated films, asdescribed in Table Dielectric life test on top of the film sample andnormal to the brass plate (the film sample was sandwiched between thebrass plate and an'end of the brass rod). The rod served as the groundelectrode. The end of the rod touching the film sample-was rounded offat a radius of curvature of The pressure exerted on the film was dueonly to the gravitational forces on the rod. The entire test apparatuswas set up in air. Sufiicient voltage was applied to the plate to give avoltage stress of 1,000'volts per mil across the sample. rapid increasein the flow of current between the brass plate and brass rod. At themoment an arc was struck between the electrodes, the are passing throughthe hole in the film caused by the failure. Theabrupt increase in theflow of current was used to trip a relay giving a record of the failure.Ten samples were treated simultaneously. The time to the failure of the"fifth sample was used as a measure of the dielectric life (under coronadischarge conditions) of the material tested. As employed herein,dielectric life of a polymeric time that a par ticular film may besubjected to a particular voltage stress under conditions of coronadischarge before actual physical breakdown, i.e., rupture, of thedielectric.

* TABLE 1 1 Dielectric life (corona resistance) of polyethyleneterephwith resin/finely Dielectric Life- Wt. Ratio Overall Ball Mill at1,000 Volts/ Example No. Coating of Gauge Time Mil (Time to Compo-(Thiclmess) ,(Hours) fifth failure 'of nents (Mils) lo-samples) (Hours)PART I0.5 MIL POLYLTHYLENE TEREPHTHALATE FILM None 0. 5 7. 5"Snper-Beckacite 1 1003/Estersil 80/20 0. 84 '168 36 GlyptaY' 2502lEstersil 2 80/20 1. 04 72 162 Glyptal" 2502 /C8b-OSil 70/30 1. 28 16851 Epon 1009 EstersilL 80/20 1. 67 240 94 PART 1I1.5 MIL POLYETHYLEN EFILM 5 "GlyptaY 2502 =/"Oab-o-si1" 70/30 2. 14 168 155 Control N oCoating 1. 5 25 1 Super-Beckacite l003-phenol-formaldehyderesin-Reicholc. Chemical Co.

2 Estersil-desoribed in 17.8. Patent 2,657,149.

3 Glyptal 2502-alkyd resin- 4 Cab-O-Sil"finely divided General ElectricCompany. silicaG. L. Cabot, Inc.

' Epon"epoxide resin-Shell Chemical Company.

EXAMPLE 5 The following formulation:

Parts Glyptal 2502 solvent solution-glyceryl phthalate having highhydroxy content-General Electric Company) 110.6 Toluene 109 Cab-O-Sil(finely divided silica-Godfrey L.

Cabot, Inc.)

27 jected to high voltage stresses.

film and the temperature at which the corona life of the film wastested; All of the films listed to the dielectric life test as describedFailure of the sample was indicated by a' current flowed,

a 40% by weight of a cured ultimate average particle size I claim: 1. Anelectrical insulating material comprising a polymeric thermoplasticdielectric base film having an adherent continuous coating consistingessentially of from 95 %v to 40% by weight of (A) a cured resinouscompound selected from the group consisting of (1) phenol-formaldehyderesins, (2), alkyd resins resulting from the interreaction of apolybasic carboxylic acid and a polyhydric alcohol (3) long chain fattyacid-modified alkyd resins, and (4) the condensation product ofepichlorohydrin and his p,p'-hydroxyphenyldimethylmethane and from 5% to60% by weight of (B) finely divided particles from the group consistingof amorphous silica and estersils, said particles having an ultimateaverage particle size between 5 millimicrons and 5 microns.

2. The product of claim 1 wherein the base film is polyethyleneterephthalate film;

3. The product of claim '1 wherein the base film is balanced, heat-set,polyethylene terephthalate film.

4. An electrical insulating material comprising polyethyleneterephthalate base film having an adherent, continuous coatingconsisting essentially of from 95% to 40% by weight of a cured,phenol-formaldehyde resin and from 5% to 60% by weight of finely dividedparticles from the. group consisting of amorphous silica and estersils,said particles having an ultimate average particle size between '5,millimicrons and 5 microns.

5. The product of claim 4 wherein the base film is balanced, heat-set,polyethylene terephthalate film.

6. An electrical insulating ethylene terephthalate base film having anadherent, continuous coating consisting essentially of from 95 to alkydresin resulting from the inter-reaction of a polybasic carboxylic acidand a polyhydric alcohol and from 5% to 60% by weight of finely dividedparticles from the group consisting of amorphous silica and estersils,said particles having an ultimate average particle size between 5millimicrons and 5 microns.

7. The product of claim 6 wherein the base film is balanced, heat-set,polyethylene terephthalate film.

8. An electrical insulating material comprising polyethyleneterephthalate base film having an adherent, continuous coatingconsisting essentially of from 95% to 40% by weight of a cured higherfatty acid-modified alkyd resin of a polybasic carboxylic acid and apolyhydric alcohol, andfrom 5% to 60% by weight of finely dividedparticles from the group consisting of amorphous silica and estersils,said particles having an between 5 rnillimicrons and 5 microns.-

9. The product of claim 8 wherein the base film is balanced, heat-set,polyethylene terephthalate film.

material comprising poly-,

10. An electrical insulating material comprising polyethyleneterephthalate base film having an adherent, continuous coatingconsisting essentially of from 95% to 40% by weight of the curedcondensation product of epichlorohydrin and bisp,p'-hydroxyphenyldimethylrnethane, and from 5% to by weight of finelydivided particles from the group consisting of amorphous silica andestersils, said particles havingan ultimate average particle sizebetween 5 millimicrons and5 microns.

11. The product of claim 10 wherein the base film is balanced, heat-set,polyethylene terephthalate film. 12. A process for coating non-fibrouspolyethylene terephthalate film which comprises applying to said film adispersion in a solvent of 10-60% solids, said solids consistingessentialy of from to 40% by weight of (A) a cured resinous compoundselected from the group consisting of (1) phenol-formaldehyde resins,(2) alkyl resins resulting from the interreaction of a polybasiccarboxylic acid andia polyhydric alcohol, (3) long chain fattyacid-modified alkyd resins, and (4) the condensation product ofepichlorohydrin and bis p,p'-hydroxy phenyldimethylmethane and from 5%to 60% by weight of (B) finely divided particles from the groupconsisting of amorphous silica and estersils, said particle size betweenS millimicrons and 5 microns; driving otf said solvent to dry the coatedfilm; and curing the coated film.

13. A process as in claim 12 wherein the base film is balanced,heat-set, polyethylene terephthalate film.

14. A process as in claim 12 wherein said cured resinous compound is aphenol-formaldehyde resin.

15. A process as in claim 12 wherein said cured resinous compound is analkyd resin resulting from the interreaction of a polybasic carboxylicacid and a polyhydric alcohol.

16. A process as in claim 12 wherein said cured resinous compound is along chain fatty acid-modified alkyd I'CSll].

17. A process as in claim 12 wherein said cured resinous compound is acondensation product of epichlorohydrin and bisp,p-hydroxyphenyldimethylmethane.

References Cited in the file of this patent UNITED STATES PATENTS2,653,891 Gentle et a1. Sept. 29, 1953 2,657,149 Iler Oct. 27, 19532,733,523 Beacham Jan. 31, 1956 2,741,568 Hayek Apr. 10, 1956 2,751,316Philips June 19, 1956 2,753,316 Campbell July 3, 1956 2,760,941 IlerAug. 28, 1956

12. A PROCESS FOR COATING NON-FIBROUS POLYETHYLENE TEREPHTHALATE FLIMWHICH COMPRISES APPLYING TO SAID FLIM A DISPERSION IN A SOLVENT OF10-60% SOLIDS, SAID SOLIDS CONSISTING ESSENTIALY FO FROM 95% TO 40% BYWEIGHT OF (A) A CURED RESINOUS COMPOUND SELECTED FROM THE GROUPCONSISTING OF (1) PHENOL-FORMALDEHYDE RESINS, (2) ALKYL RESINS RESULTINGFROM THE INTERREACTION OF A POLYBASIC CARBOXYLIC ACID AND A POLYHYDRICALCHOL, (3) LONG CHAINS FATTY ACID-MODIFIED ALKYD RESINS, AND(4) THECONDENSATION PRODUCT OF EPICHLOROHYDRIN AND BISP,P''-HYDROXYPHENYLDIMETHYLMETHANE AND FROM 5% TO 60% BY WEIGHT OF (B)FINELY DIVIDED PARTICLES FROM THE GROUP CONSISTING OA AMORPHOUS SILICAAND ESTERILS, SAID PARTICLE SIZE BETWEEN 5 MILLIMICRONS AND 5 MICRONS,DRIVING OFF SAID SOLVENT TO DRY THE COATED FLIM, AND CURING THE COATEDFLIM.